Sequestration of Reactive Orange 16 Dye by Nitrogen‐Impregnated <i>Terminalia arjuna</i> Seed Biochar: Insights into Kinetics, Thermodynamics, Reusability, and Phytotoxicity Assessment

Abstract The present investigation deals with feasibility to use available biomass waste, Terminalia arjuna seed as cost‐effective sorbent for reactive orange 16 (RO16) dye elimination from wastewater. The present study centers around the modification of Terminalia arjuna seed biochar (TASB) by applying one‐step synthesis procedure, by using melamine as a nitrogen doping agent and phosphoric acid (H 3 PO 4 ) as chemical activator. The effectiveness of the modified nitrogen‐doped Terminalia arjuna seed biochar (ND‐TASB) as sorbent was assessed on RO16 dye removal from aqueous medium. Surface morphology of synthesized sorbent was identified by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). Modified ND‐TASB reflected enhancement in adsorption efficacy, at pH of 4, maximum removal of RO16 dye 95% was observed. By utilizing TASB and ND‐TASB as an adsorbent, elimination of RO16 dye from aqueous medium was examined under varying experimental conditions like pH, adsorbent amount, and exposure period were reported 1.5 and 1 g at 40, 60 min at pH of 4, respectively with RO 16 dye (200 mg/L concentration). Isotherm and kinetic results were appropriate with Langmuir isotherm and pseudo second‐order models. Maximum sorption efficiency of TASB was 2.51 mg g −1 , whereas NB‐TASB exhibited 4.84 mg g −1 , that is, 93% enhancement. Alterations in free energy (Δ G °), enthalpy (Δ H °), entropy (Δ S °) of adsorption process were analyzed to assess nature of adsorption phenomenon. The negative values of Δ G ° confirms that adsorption phenomenon was spontaneous and feasible. Enthalpy value was −60.39 kJ mol −1 whereas Δ S ° had 182.72 J K −1 for ND‐PKSB. Phytotoxicity study revealed that TASB or ND‐TASB treated RO16 dye solution enhanced pearl millet seedlings growth and biochemical variables with better growth with ND‐PKSB treated RO16 dye solution. Moreover, the regenerated adsorbents exhibited high efficiency for RO16 removal up to five consecutive cycles. These findings establish that modified ND‐TASB acts as durable, multifaceted, and recyclable environmentally benign sorbent with superior properties for its application at industrial scale.